In many applications, computer-aided geometry modeling imposes high requirements on the methods and algorithms used. This applies to the description of regular geometries and freely shaped areas alike. At the same time, the close integration of the "naked" geometric description with other aspects, such as functionality, material properties, tolerances, etc., is of great significance for many users. To meet this demand, both research and many software companies developed concepts by means of which the necessary improvement is said to be achieved in the representation capabilities. A further increase in the performance of CAD/CAM systems and of the geometry modeling contained therein shall be achieved by a close integration of geometry and knowledge based processing techniques (cf. Krause et al., Feature Processing as Kernel for Integrated CAE Systems, IFIP International Conference, May 1994, Valenciennes). Information units (features) are a key concept for this integration: They represent a kind of bundling of geometry and geometry-related knowledge.
The CSG method (Constructive Solid Geometry), the B-Rep method (Boundary Representation) and various concepts of feature modeling e.g., the EREP method (cf. Hoffmann et al., EREP--an Editable, High-Level Representation for Geometric Design and Analysis, Technical Report, Purdue Univ., West Lafayette, 1994) or Rieger, E.: Semantics-Oriented Features for Continuous Support of Product Design, Hanser Verlag, Munich, 1995! have already been known for geometry modeling.
From the viewpoint of a knowledge-based system, the main problem of the CGS modeling is the history-based (sequence-dependent) description of the geometry.
Due to missing information, the Boundary Representation Method is only partly able to be integrated with knowledge-based techniques. This information comprises the topological and generic area-related information and the relationships between them, but not the relationships with volumes and volume-related features.
The EREP method currently represents one of the most expressive feature concepts. It is based on the idea of providing a representation of geometric shapes which exists independently from the "actual" geometric calculation in a geometry modeler and as such can, e.g., also be edited. This is achieved by two central starting points:
The provision of a formal description language, in which the feature and consequently also the geometry can be defined in such a way that is independent from a geometry modeler. This mode of representation is based on a sweeporiented geometry modeling; features can be described by establishing "from-to" relationships between them and already existing features (more precisely their areas or faces). To achieve the necessary flexibility of the representation, degrees of freedom can be utilized in the geometry modeling in this prior-art method due to the presence of special geometric relationships (constraint techniques). PA1 The expression capabilities are often too weak in the feature representations: They are either based on volume-oriented, part-oriented or sweep-oriented geometry modeling representations, and offer too few representation capabilities for the other "modes of view." The of defining relationships between different features and of building up a hierarchy of features are often too weak. PA1 Feature interactions cannot be described with the necessary expressiveness. This applies especially to design-oriented feature concepts, while greater importance is usually attached to this aspect in concepts for the manufacturing feature. PA1 It is especially problematic from the viewpoint of knowledge representation that most feature concepts are still based on a procedural approach. They are more or less a kind of "macro," with which a predefined sequence of instructions can be given for the geometry modeler and these instructions can be activated. PA1 volume level as the basic level for the description of elementary structures, whose composition is permanently preset, PA1 topology level as a representation of the overall geometry, PA1 a generic area level located between the volume level and the topology level, and a generic edge level likewise located between the volume level and the topology level for establishing the relationship between the topological structures and the volumes generating them.
The goal is to utilize independent possibilities of the geometric description as well as the integration of constraint methods in geometry modeling.
The feature modeling in the prior-art method is based on a procedural approach, while no attention is paid at first to the declarativity of the representation of the geometry. The integration of different aspects of geometry modeling, namely, volume-related, area- and topology-relevant information, in an integrated manner and with regard to the treatment of a mutual dependence is missing.
The above-described feature concepts satisfy only some of the requirements to be imposed on an integration with knowledge-processing systems. The essential weak points can be summarized as follows:
Both the question of the existing possibilities of expression and a declarative description are essential for a feature concept that is to act as a link with the knowledge representation. Possibly all relationships in geometry and between geometry and other aspects (such as material, tolerances, etc.) must be able to be adequately described, and this must be able to be done in a declarative manner, in which the significance of the description arises from itself, regardless of the underlying processing model.
Prior-art types of geometry modeling lead to the circumstance that neither of the above-mentioned two goals, namely, possibilities of expression and declarativity, can be achieved to the necessary extent. The use of volume- or area-oriented geometric description limits the representation of relationships of the other "level," and the sequential (sequence-based) form of the modeling of geometric shapes prevents a declarative approach. This procedure also collides with the necessary interactive mode of operation, in which not only does the designer build up geometry step by step, but also discards, revises, modifies it, etc. Based on the previously compulsory sequence-based geometry operations, the designer frequently performs geometry operations which are not particularly intuitive: To avoid a lengthy setback in the sequence of operations performed in the case of a necessary revision of the geometry, new geometry operations are attached, and even though these geometry operations ultimately bring about the same overall geometry as would have been caused by the revision of earlier steps, their meaning is understandable in this context only.
In addition, the loss of information, which is the result of the representation of the geometries of a Boundary Representation, frequently causes problems, e.g., when obtaining manufacturing-relevant representations (NC programming) from the geometry or when aspects of manufacturing are taken into account during the design process (in the sense of a Concurrent Engineering).